Because of the skin-effect losses in a long transmission line, high-rate data transmission is markedly attenuated after traveling along a long line. Consequently, when two transmitters are attempting to communicate with each other on a transmission line when they are at a substantial distance apart, they would encounter great difficulty because of the attenuation. Further, should the two transmitters broadcast simultaneously, data collision occurs; that is, the high rate data being transmitted by each of the transmitters at either end of the transmission line interferes with the other to produce a nearly unintelligible transmission, or gibberish. In the prior art, neither receiving end would be aware that such a collision of data is taking place. This problem is further compounded with the simultaneous broadcasting by several transmitters; the data collision would be correspondingly more severe.
Since direct current (DC) in a transmission line is attenuated the least, the method in accordance with the present invention employs this fact to indicate the presence or absence of a transmission. DC drivers are switched on coincidentally with the beginning of any transmission and off at the end. Hence, with the presence or absence of a DC level, the receiving station, or node, on the transmission line can determine whether data transmission is being propagated along the line or not and can then react accordingly.